ASUS RT-AC66U Wireless 802.11ac Router Detailed Features
The top cover of the RT-AC66U is not easily removed, even after you remove four screws that are hidden underneath the rubber feet, towards the four corners of the unit. There are plastic latching tabs all around the perimeter, which took a bit of force to release, but none of them broke during the process. The regular user won’t notice, since there’s no practical reason for removing the top cover during normal use. It’s not like it’s a PC Gaming Case, for instance. A single PC board fills the entire space inside the enclosure, and there are no metal plates or other ballast inside, to add some weight. Like almost all routers and switches intended for home use, the RT-AC66U is lightweight enough to get thoroughly bossed around by a small group of unruly Ethernet cables. Looking at the internal layout, I can’t help but wonder if the cooling could be vastly improved by putting the component side of the board, along with the big heatsink, on the reverse side of the enclosure. That’s where all the vents are. The only thing that really needs to face the front is the block of LED indicators.
Once the main PC board is exposed, the first thing that stands out is the size of the aluminum heat sink that runs almost the full width of the board, which is almost as wide as the case. It’s tough to see in the pictures, but it’s actually tapered from back to front, which makes it a very custom part. Along the top edge are all the I/O points, most of which are soldered directly to the board. The antenna connectors are different, since RF signals are a bear to control and contain. They have shielded coaxial cables making their way from the rear panel SMA connector bodies, out to RF “safe zones”, spaced out around the perimeter of the active circuitry. Peeking out from below the central heatsink is an aluminum RF shield, commonly called a “can”, because it completely encloses a section of the active circuitry. If you’ve ever ripped apart an old cell phone, you’ll be familiar with the construction details of the shields. Nestled in between the 1000BASE-T LAN and WAN connectors and the switching hardware are several isolation and impedance matching pulse transformers that keep the Ethernet signals flowing smoothly, with minimal noise. The plastic mounting block along the bottom edge, for the LED lenses, makes sure the bright blue lights get piped to the front panel accurately.
Once the heatsink is removed, the two basic functional blocks are revealed. Although it’s impossible to see any details through the RF shielding cans, the FCC has generously shared the photos of their full tear-down with the world, and I can tell you what’s inside each of them. On the left are the two software-defined radios for the 2.4GHz and 5GHz bands, and their associated RF power amplifiers. The star of the show here is the 5GHz radio, sourced from Broadcom. Their BCM4360 Wi-Fi chip sits at the top of their “Fifth Generation” line of Wi-Fi devices. This IC is one of the things that set this router apart from the pack, by generating the three 802.11ac data streams that give you that massive 1.3Gbps maximum data rate. The 2.4 GHz chip is the Broadcom BCM4331, which is the same device that ASUS used on the RT-N66U router, to good effect with 802.11n. With the heatsink in place, the location of the antenna connections seemed a bit random, but you can now see how they tap into the RF section of the board from three different locations. The E-shaped component layout at each pickup point is there because there are two RF signals being pumped into each antenna, one in the 2.4GHz band and one in the 5GHz band. The top and bottom of the “E” are the entry points for each of the two signals, and the center is the summing point.
The next picture is courtesy of the FCC, and I apologize for the obvious barrel distortion in the image. However, I certainly appreciate the fact that I don’t have to destroy the review sample, just to get a peek at the high tech chips inside. Trust me; it’s almost impossible to get those aluminum lids off without destroying them, and usually something else on the board at the same time. The right hand section, with its own shielding enclosure, contains the brains of the router. Inside the can are the CPU, 256 MB of Samsung DRAM, and the switching IC. The RT-AC66U uses the same CPU as the RT-N66U model, a Broadcom BCM4706. It contains a 600MHz MIPS core and communicates with the Wi-Fi radio chips via two separate PCIe interfaces. The switch IC is also from Broadcom and it’s a BCM53125, which complies with the IEEE 802.3az Energy Efficient Ethernet (EEE) standard.
While the majority of the ICs and the overall architecture are sourced from Broadcom, I wouldn’t call the board a reference design, by any stretch. There are major differences in hardware design and layout between this device and all of the competitor’s products. To me, the RT-AC66U looks like a more mature, and finished design than the others. Several of the others have their 5GHz radios mounted on a mini-PCIe module that plugs into the main board, and their Radio ICs are not even shielded. I realize the IEEE 802.11ac standard isn’t ratified yet, but I’m willing to bet that any needed modifications could be implemented in firmware updates. Even though the RT-AC66U came onto the market very quickly relative to other 802.11ac products, ASUS didn’t compromise the design process.
The bottom of the PC board looks rather sparsely populated, until you realize that all those tiny little specks are actually surface mount components. Some of the power supply parts, VRMs and capacitors mostly, are also in evidence here and they’re easier to see. The closer these components are to the electrical load that they service, the more effective they are. It just so happens that the back side of the PCB is the absolute closest location in many instances. The 128MB of Samsung flash memory is located on the back side, as well; this is where the firmware that drives the CPU is stored. The metal plate that’s mounted to the back side of the PCB assists mostly with shielding purposes; there’s not much heat that gets transferred to it or through it. All in all, the backside of this PCB is nowhere near as interesting or complex as the front side.
Normally, I like to dig down one more layer, to the functional block level where the technology really starts to get interesting. But Broadcom is pretty tight-lipped about the technology that’s inside their chips. They aren’t like Intel, where they have such a monopoly on the market that they can afford to tell the whole wide world what they’re doing now, and what they’re planning for the next five years. So we’ll skip the hard core engineering stuff this time, and take a look at the Features and Specifications for the ASUS RT-AC66U wireless gigabit router.